The present invention relates to an overtone crystal oscillating circuit including a complementary MOS inverter and a crystal resonator. FIG. 1 shows a conventional crystal oscillating circuit having a CMOS inverter 2 and a crystal resonator 4 connected between input and output terminals of the CMOS inverter 2. The CMOS inverter 2 is comprised of P and N channel insulated gate type MOS transistors 6 and 8 of which the current paths are connected in series between a positive power source terminal V.sub.D and a reference power source terminal V.sub.S which is grounded, for example. A DC feedback resistor 10 is connected between the input and output terminals of the CMOS inverter 2. A variable capacitor 12 and a capacitor 14 are connected between the input terminal of the CMOS inverter 2 and ground. A capacitor 16 is connected between the output terminal of the CMOS inverter 2 and ground.
In the crystal oscillating circuit of FIG. 1, the crystal oscillating resonator 4 and capacitors 12, 14 and 16 cooperate to constitute a resonance circuit which is driven by a drive circuit including the CMOS inverter 2 and resistor 10 to perform an oscillating operation. It is often required to obtain a high oscillating frequency using the crystal oscillating circuit. However, the maximum value of the oscillating frequency of the crystal oscillating circuit is limited to about 14 MHz due to restrictions such as a restrictive factor in manufacturing of the crystal oscillating resonator 4, delay time of CMOS inverter 2, and a limit of the gain.
For this reason, in order to construct a crystal oscillating circuit for providing a frequency higher than 14 MHz, it has been a common practice to use an overtone crystal oscillating circuit including, as shown in FIG. 2, a bipolar transistor 18, a crystal oscillating resonator 20 and a circuit 22 which, together with the crystal oscillating resonator 20, forms a resonance circuit and includes a number of tuning passive elements to apply a bias voltage to the bipolar transistor. Thus, the crystal oscillating circuit performs the oscillating operation at an odd harmonic of the fundamental frequency of the crystal resonator.
Thus, the conventional overtone oscillating circuit of this kind is disadvantageous in that a number of circuit elements are used, increasing the cost and resulting in susceptibility to variation of the power source voltage and poor reliability of the operation. A MOS process, which has dominantly been used for manufacturing modern semiconductor devices, is not applicable for manufacturing the oscillating circuit shown in FIG. 2. Therefore, it is impossible to form the oscillating circuit shown in FIG. 2 on the same chip of a semiconductor device manufactured by the MOS process.